1. Field of the Invention
The present invention is related to a bi-directional dc/dc power converter having a neutral terminal. More particularly, the present invention is related to the bi-directional dc/dc power converter for bi-directional power flow. When the power converter acts as a step-up power converter, the power flow direction is from the low voltage to the high voltage, and two dc voltages with the same amplitude are serially connected to the neutral terminal to provide a double voltage at the output of the second voltage terminal set. When the power converter acts as a step-down power converter, the power flow direction of the power converter is inverted, and the power flow direction is from the high voltage to the low voltage. The output voltage at the first voltage terminal set is half of the voltage at the second voltage terminal set.
2. Description of the Related Art
Referring to FIG. 1, a conventional DC/DC power converter circuit comprises a DC/DC step-up power converter 1. The DC/DC step-up power converter 1 includes an input terminal 11, a power electronic device 12, an output terminals 13, an inductor 14, a diode 15 and a capacitor 16. As an input dc voltage is applied to the input terminal 11, the input dc voltage provides energy to the inductor 14 during the interval when the power electronic device 12 is turned on. During the interval when the power electronic device 12 is turned off, the inductor current transfers its stored energy to the capacitor 16 via the diode 15 so that the capacitor 16 may provide with an output dc voltage larger than that of the input dc voltage for the output terminals 13. Generally, the output terminals 13 of the DC/DC step-up power converter 1 has only two output points and no neutral terminal that is used to connect two dc voltages with the same amplitude in series to provide a double voltage. Accordingly, the DC/DC step-up power converter 1 only allows unidirectional power flow. However, in many applications, the DC/DC step-up power converter 1 requires a neutral terminal at its output terminals and bi-directional power flow direction.
For example, a single-phase half-bridge inverter must employ a neutral terminal at its dc side. The DC/DC step-up power converter 1 is used to supply a dc voltage that is double of the voltage across two dc capacitors of the single-phase half-bridge inverter. However, there is no neutral terminal in the output terminals of the step-up DC/DC power converter 1. Thus, the step-up DC/DC power converter 1 cannot supplies two dc voltages with the same amplitude for the single-phase half-bridge inverter. If the amplitude of both dc capacitor voltages is not equal, the performance of single-phase half-bridge inverter will be degraded. Additionally, the power flow in the step-up DC/DC power converter 1 is unidirectional. However, bi-directional power flow is required in many applications of the half-bridge inverter, such as the single-phase parallel operation uninterruptible power supply (UPS) configured by the half-bridge converter. In this UPS circuit topology, it only uses a half-bridge inverter to convert the power from ac to dc for charging the power into the battery and supplying the dc power of UPS for converting the power from dc to ac to the load. The bi-directional power flow is required in the DC/DC power converter according the power flow of half-bridge inverter for charging the battery and regulating the dc voltage of the half-bridge inverter.
Another example, a three-phase four-wire inverter with a leg consisting of two separated dc capacitors also desires a DC/DC power converter with a neutral terminal at the output terminals to supply two dc voltages with the same amplitude for normal operation. Moreover, the bi-directional power flow for DC/DC converter is also required in the three-phase four-wire parallel operation UPS. Hence, the DC/DC step-up power converter 1 cannot satisfy the requirement in this UPS circuit topology that uses the three-phase four-wire inverter with a leg consisting of two separated dc capacitors.
The above description indicates that the DC/DC step-up power converter 1 fails to provide with a neutral terminal at its output terminals and cannot supply two dc voltages with the same amplitude. Besides, it cannot carry out the bi-directional power flow direction. Consequently, the application of the DC/DC step-up power converter 1 is limited.
The present invention intends to provide a bi-directional dc/dc power converter having a neutral terminal and carrying out bi-directional power flow. When the power converter acts as a step-up power converter, the power flow direction is from the low voltage to the high voltage, and two dc voltages with the same amplitude are serially connected to the neutral terminal to provide a double voltage at the output of the second voltage terminal set. When the power converter acts as a step-down power converter, the power flow direction of the power converter is inverted, and the power flow direction is from the high voltage to the low voltage. The output voltage at the first voltage terminal set is half of the voltage at the second voltage terminal set.